Purpose:
To compare in guinea pig eyes, different experimental methods used to quantify tear film changes associated with age and experimental eye dryness (DE).

Methods:
Guinea pigs of both sexes and different ages were studied. In one group of animals the main lacrimal gland was surgically excised (DE animals). Tearing rate was measured using phenol red threads that were placed in the nasal canthus for 30 s, (tear secretion= mm of wet thread). To determine the tear break-up time (TBUT) 1µl of 2% fluorescein was instilled on the conjunctival sac; lids were closed manually 3-4 times; after that, with the eye open, time to break-up the tear film was measured in seconds. Corneal surface temperature was measured using an infrared thermal camera (InfRec R300SR, Nippon Avionics). Temperature was measured continuously in the full open eye during a period of at least 10s, after keeping the eye closed manually during 3 s. Images were analyzed using the instrument's software.

Results:
Tear secretion was significantly higher in old (15 months) compared with young (2 months) animals (12.7±1.0 mm vs 19.3±1.5 mm; p<0.001; at 2 and 15 months of age respectively). No differences were observed in the TBUT (2.7±0.1 s vs 2.1±0.1 s). The value of corneal surface temperature measured immediately after the eye opening was higher in older guinea pigs (35.2±0.20C vs 36.9±0.20C; p<0.001; at 2 and 15 months of age respectively). In DE animals tear secretion and TBUT were significantly lower than in controls (8.2±1.5 mm, p<0.001; 1±0 s, p<0.05, respectively). Temperature after the opening was also higher in the DE group (37.2±0.04 0C, p<0.001). The slope of the corneal temperature decrease during the 5s afterwards was faster in older and in DE animals (-0.02±0.02 0C/s, -0.07±0.040C, -0.06±0.010C/s, control 2 months, control 15 months and DE respectively, p=0.254 and 0.179, comparisons with 2 months data).

Conclusions:
Tear secretion (volume) changes in the eye surface can be detected easily and reproducibly in guinea pig using phenol red threads. TBUT was often very short, making comparisons difficult while dynamic thermography may be useful for indirect evaluation of the tear flow, as those occurring during aging or DED disease.